Hydraulic tensioners have been widely used to maintain tension in power transmission media such as timing chains, used for transmitting rotation from the crankshaft to one or more camshafts in an internal combustion engine. The tensioners not only maintain proper tension in the transmission medium, but also prevent vibration. A typical hydraulic tensioner is depicted and described in Japanese Patent Publication No. 3226037 (pages 1 and 2, FIG. 6).
As shown in FIG. 7, in a conventional hydraulic tensioner 500, a cylindrical plunger 520 is slidable in a plunger-accommodating hole 511 formed in a housing 510. A plunger-biasing spring 530, in compression between the plunger and the bottom of the hole, urges the plunger 520 in the protruding direction. A high pressure chamber R, formed by the plunger-accommodating hole 511 and the plunger 520, is filled with oil supplied under pressure from the engine through a check valve unit 540.
The high pressure oil chamber may be incompletely filled with oil when the engine is started. Under this condition, backlash noise is generated when the plunger of the tensioner retracts rapidly as a result of impact applied to the plunger by the timing chain. This rapid retracting movement of the plunger 520 can be prevented by a ratchet mechanism. In the tensioner of FIG. 7, the ratchet mechanism comprises teeth 521 formed on the plunger, a pawl 580, pivoted on a pin 570 extending from the housing 510, and a spring 590, which biases the ratchet pawl 580 so its teeth 581 approach the plunger teeth 521.
As shown in FIG. 7, when the ratchet mechanism is operating to prevent retraction of the plunger, the engagement of the pawl teeth 581 with the ratchet teeth 521 of the plunger, generates unbalanced loads F1 and F2. These unbalanced loads can cause localized wear of the housing, especially if the housing is composed of a material such as aluminum.
Furthermore, the performance of the conventional hydraulic tensioner 500, in response to a change in tension in a timing chain depends, to a significant extent, on meshing of the ratchet teeth 521 with the pawl teeth 581. Smooth meshing requires high machining accuracy in the formation of the ratchet mechanism, and especially, accurate mounting of the supporting pin 570, and accurate positioning of the pin-receiving hole of the pawl. Achievement of the required accuracy is difficult.
Thus, objects of the invention are to provide a simple retraction-limiting mechanism which reliably prevents backlash noise upon starting of an engine, to solve the above-mentioned problems encountered in the manufacture of conventional hydraulic tensioners, to simplify manufacture of the tensioner and avoid the difficulties resulting from the requirement for high machining accuracy in the formation of the ratchet mechanism, and to reduce wear of tensioner components.